Fluid filled vibration isolators are being used increasingly to mount engines and transmissions to the frames of automotive vehicles. A typical fluid filled vibration isolator includes a pair of opposed variable volume fluid filled chambers separated by a partition in which is provided an elongate arcuate inertia track passageway providing continuous fluid communication between the chambers. A decoupler is mounted in the partition and cooperates with the inertia track passageway to provide certain dynamic operating characteristics. The vibration isolator is tuned to provide various magnitudes of stiffness and damping within particular frequency and amplitude ranges to eliminate idle shake, engine bounce, noise, and like problems.
In most commercially available fluid filled vibration isolators, the dynamic operating characteristics are determined by the design of the isolator. Some have proposed fluid filled vibration isolators which are designed so that their dynamic operating characteristics can be actively controlled in response to various conditions such as vehicle and engine speeds, and the like. Examples of such mounts may be found in the following U. S. Pat. Nos. 4,415,148; 4,505,462; 4,531,484; 4,537,275; and in Japanese Published Appln. No. 57-129944. In U.S. Pat. No. 4,505,462, the compliance of one or both of the fluid filled chambers is adjusted, and the dynamic operating characteristics of the mount thereby changed, either by constricting the periphery of one of the chambers or by varying the pressure of air in a chamber confronting a flexible wall which defines a portion of another one of the fluid chambers. In German Published Appln. No. 3,244,296 a fluid filled mount is provided with a diaphragm confronting one of the fluid filled chambers, and air is contained in a chamber behind the diaphragm to affect the overall compliance of the fluid filled chamber.
Known actively controlled fluid filled vibration isolators having air chambers wherein the pressure is regulated to vary the compliance of the fluid filled chamber have certain limitations. For one thing, such isolators require a source of compressed air which may or may not be readily available on a vehicle, and which is generally not provided in most automobiles. Secondly, while air pressure regulation can vary the frequency at which minimum and maximum dynamic stiffnesses occur, as well as their magnitudes, the dynamic operating characteristics tend to change in an analog manner, i.e. gradually, thereby blurring desirable sharp distinctions in the dynamic stiffness characteristics. While there may be certain applications in which actively controlled fluid filled vibration isolators which do not have precisely adjustable dynamic stiffness characteristics may function satisfactorily, there is a need for an actively controlled fluid filled vibration isolator which can be positively adjusted to provide precisely predictable dynamic operating characteristics.